1. Field of the Invention
The invention relates to a process for hermetically sealing very-high-frequency components in a case in the form of a device comprising metal conducting crossovers.
2. Description of the Prior Art
It is known that sealed crossovers formed by glass beads cannot be used for very high frequencies because of the energy losses in the glass. Recourse is then had to crossovers between two pieces of ceramic which are for example the bottom and the side walls of a component case. Two principal cases are to be considered:
First case: The parts are made from an insulating polycrystalline ceramic for example from sintered alumina, which forms a high-grade insulator for very high frequencies. In this case, the conductors are provided for example in the form of metalizations deposited on the alumina of the bottom of the case. Generally this bottom forms a plate with dimensions greater than that of the bottom properly speaking, this latter being limited by a frame forming the side walls of the case. The metalizations extend beyond the walls and pass sealingly between frame and plate, thanks to sealing generally using glass having a high softening temperature, the metalizations being provided so as to withstand this operation without damage, for example by using a molybdenum and manganese alloy.
This method has the disadvantage of introducing glass, which even in small quantities is a source of high-frequency losses at the crossovers.
Second case: The parts are made from green ceramic before assembly of the plate and the frame forming the bottom and the edges of the case. There is in fact known the relatively delicate technology of cast, so-called plastic, alumina made from tapes having a thickness of about a millimeter (during subsequent sintering, the tape undergoes considerable shrinking and its thickness practically does not exceed a millimeter).
The plate and the frame are formed by cutting out a tape of cast alumina. To obtain a case whose edges have a height greater than a millimeter, it is then necessary to superimpose two or more tapes.
The metallization obtained by silk-screen printing of the conductors of the case is followed by a heat treatment to remove the volatile solvents of the paste to be silk-screen printed.
The parts are then assembled, maintained in contact with a light pressure, then sealed by sintering at about 1500.degree. C.
Hermetic sealing is ensured by the diffusion of alumina and metal between the contiguous parts during sintering.
The disadvantages, in the second case, result from the high cost of the technology of cast alumina, from the limitation of the height of the edges of the case when a single frame is used drawn from a tape of cast alumina, finally from the considerable shrinkage of the parts after sintering of the ceramic.
The invention tends to eliminate the disadvantages of prior methods.